1. Field of the Invention
The present invention relates to a heating device in which a heating roller heated at high temperature is brought into contact with a member to be heated, and more particularly to the heating device used in a fixing process of a toner image in an electrophotographic printing process and an image forming apparatus, such as a printer, a copier, etc., having the heating device in a fixing section thereof.
2. Discussion of the Background
In an electrophotographic printing process, an electrostatic latent image formed on a surface of a photoconductive element is developed with toner. The developed toner image is transferred onto a transfer sheet. Then a fixing process is performed to fix the toner image onto the transfer sheet. In the fixing process, the toner image is fixed onto the transfer sheet by at least heat and pressure. In a fixing device, the transfer sheet having the toner image is conveyed while being sandwiched by a fixing roller and a pressure roller. The fixing roller is heated to a high temperature. The pressure roller is driven while being press-contacted with the fixing roller. The toner image on the transfer sheet is then fixed while the transfer sheet is conveyed by the fixing roller.
In recent years, an attempt has been made to shorten a period of time required for a warm-up operation and to process an image at high speed in an image forming apparatus, in which an electrophotographic printing process is performed, such as a printer, a copier, etc. Corresponding to the above-described attempt, an attempt has been made to heat a fixing roller to higher temperature and convey a transfer sheet at higher speed in the fixing device. A resinoid sliding bearing, which is obtained at low cost, may be employed as a bearing of the fixing roller. However, a disadvantage to the sliding bearing is that a driving torque increases in the early stages of use and especially over the lapse of time, and that its durability decreases due to a deformation caused by a heat stress, when the fixing roller rotates at high speed.
Thus, an attempt has been made to use a ball bearing instead of the sliding bearing. The driving torque decreases when the ball bearing is used. A motor, which is smaller than that is used when the sliding bearing employed, is then used, resulting in a reduction in costs. Because the fixing roller is formed into a drum shape having a heater inside, it is advantageous that the fixing roller has a decreased thickness to shorten a period of time required for a warm-up operation. However, if the thickness of the fixing roller is decreased, a large amount of deformation appears. Thus, a wrinkle of a transfer sheet tends to occur.
To obviate the above-described inconvenience, a value of “thickness/outer diameter” of an image region (i.e., a central portion of the fixing roller through which a transfer sheet is conveyed) of the fixing roller needs to be in a predetermined range portion of the fixing roller, which is supported by a bearing provided to a main body of the apparatus, is smaller than that of the image region thereof.
When a ball bearing is employed to shorten a warm-up time and process an image at high speed, heat is conducted to the ball bearing if the ball bearing directly contacts a core metal of a fixing roller. Thus, the ball bearing is heated to high temperature, resulting in a decreased lifetime of the ball bearing. Further, during a warm-up operation, heat dissipates from the both ends of the fixing roller which are supported by the ball bearing. Thus, a temperature of the end portions of the fixing roller tends to decrease, resulting in a faulty fixing. Therefore, an insulating resin layer (i.e., a bush) or a ceramic member is provided between the fixing roller and the ball bearing to obviate the above-described behavior of heat, thereby preventing the above-described inconvenience. However, a noise is produced when the resin layer or ceramic member is rubbed against the core metal and ball bearing if the resin layer or ceramic member is conventionally provided as a separate part. The production of the noise may be reduced using a grease, however, a fluorine grease for a high-temperature is expensive. When the ceramic member is used, an insulating effect is enhanced, however, resulting not only in an increase in costs but in a further production of the noise.
The below-described problem needs to be solved when the ball bearing is adopted. As described above, the thickness of the fixing roller is determined such that the value of “thickness/outer diameter” of the image region is in the predetermined range. However, the thickness of the supported portion of the fixing roller needs to be adjusted based on the determined thickness of the image region of the fixing roller to obviate a deformation of the fixing roller and an occurrence of a wrinkle of a transfer sheet. Conventionally, an inner diameter of the supported portion of the fixing roller is set to a value equal to that of the inner diameter of the image region. Therefore, when a design change is made in the thickness of the supported portion, an outer diameter of the supported portion is eventually changed.
Thus, a general-purpose ball bearing is not used, and a customized ball bearing that fits to the modified outer diameter of the supported portion of the fixing roller is required. Conventionally, the outer diameter of the supported portion is smaller than that of the image region. The supported portion is then supported by a bearing provided to a main body of the apparatus. Thus, a replacement of the fixing roller with new one is not easily performed without dismantling a relating component.